Power supplies in many electronic devices output a DC voltage. Generally, the DC voltage is derived from an AC power source. The AC voltage is rectified into an unregulated DC voltage by a rectifier bridge. The unregulated DC voltage is converted into the DC voltage as needed by a switching power supply.
A transformer or inductor is usually used as a tank element in the switching power supply. For example, in a flyback converter a transformer is used. A switch is electrically coupled to the primary winding of the transformer. The switch is turned on and off so as to alternately store energy in the transformer and transfer the stored energy to the secondary winding of the transformer. An output capacitor is electrically coupled to the secondary winding of the transformer and a rectified voltage is generated thereon. The rectified voltage provides the DC output voltage of the switching power supply. The DC output voltage increases and decreases inversely with the load. The heavier the load, i.e. the higher the output current, the lower the output voltage, and vice versa. Generally, the DC output voltage is fedback to compensate for the variation of the load.
Under CCM (continuous current mode—which means the current flowing through the tank element is continuous), the output power of the switching power supply is
            P              out_        ⁢        CCM              =                  1        2            ⁢              L        ⁡                  (                                    I              peak              2                        -                          I              valley              2                                )                    ⁢      f      ⁢                          ⁢      η        ,while under DCM (discontinuous current mode—which means the current flowing through the tank element is discontinuous), the output power is
            P              out_        ⁢        DCM              =                  1        2            ⁢              LI        peak        2            ⁢      f      ⁢                          ⁢      η        ,wherein L is the inductance of the tank element, Ipeak is the peak value of the current flowing through the tank element, Ivalley is the valley value of the current flowing through the tank element, f is the switching frequency and η is the efficiency of the switching power supply.
One prior art method for controlling the switching power supply is to maintain the switching frequency constant while regulating the peak current, such as the peak current control. Another method is to maintain the peak current constant while regulating the switching frequency, such as the off time control. In the first method, the efficiency of the switching power supply will be greatly decreased at light load because of the fixed switching frequency. In the second method, the switching frequency may be decreased to be too low (below 20 KHz) at light load and an audible noise is generated.